The present invention relates to a digital image scan converter or D.I.C. It more specifically relates to a process for the insertion of magnified insets into the image supplied by such a D.I.C., as well as to the D.I.C. for performing this process.
The essential function of a D.I.C. is to convert an image supplied in polar coordinates and which is relatively slowly renewable into a bright television-type image or picture, thereby permitting its exploitation in an illuminated ambient. This slowly renewable image is generally a radar image, but it can also be an image from a sonar, an infrared sensor, an echo sounding system or the like, which can be displayed on a screen operating in the television mode.
FIG. 1 shows the block diagram of a D.I.C. in the case of a radar. The D.I.C. comprises the following components:
A television monitor 7 on which the radar information is displayed.
An image memory 4 containing in digital form the image to be displayed on the monitor 7. The capacity of memory 4 is adapted to the television standard used, i.e. it must have the same number of storage positions as the image displayed on the television monitor has points (or pixels). In addition, the brightness of each point is coded by means of a certain number of bits, e.g. three bits authorizing eight brightness levels for each point. The reading phase of the content of memory 4 intended for the television monitor 7 and the phase of writing into the memory the radar information supplied by an assembly 2 are asynchronous. Reading has priority and writing is stopped during a reading phase.
An assembly 1 of circuits is provided for processing the radar video signals received by the D.I.C. It is pointed out that analog video signals are constituted on the one hand by a sync signal (S.sub.Y) indicating that a pulse has been emitted by the radar, and on the other hand by the actual video (.rho.), constituted by all the responses (echos) to said pulse. Assembly 1 in particular ensures the conversion of the incident analog video signal into digital form signal, the processing of said signal, whose details are variable as a function of the radar type, and the storage of the video information corresponding to each radar pulse, called radial sweep.
An assembly 3 of coordinate conversion circuits is needed because the radar information, which is generally supplied in polar coordinates, has to be written onto the television screen 7 in Cartesian coordinates. For this purpose, assembly 3 receives the signals representing the rotation of the radar antenna, i.e. indicating at all times the angular position of the beam of the radar (radar antenna) with respect to a reference direction, generally North. The rotation signals are on the one hand constituted by a north signal (N), which is a pulse signal supplied during each passage of the beam through North and on the other hand by an angle increment signal (.epsilon.) indicating that the beam has turned by an nth of a turn with respect to the preceding increment, if n increments correspond to 360.degree.. Thus, assembly 3 ensures the addressing of the image memory 4, whereby the processing assembly 1 supplies via an assembly 2, the information to be written into the image memory 4, synchronously with the addressing.
An assembly 5 of television mode reading circuits ensures the read addressing of the image memory 4.
An assembly 6 of television mode output circuits receives the information stored in the image memory 4 and as addressed by assembly 5, ensures the digital-analog conversion to produce the television video signal intended for monitor 7, as well as the generation of conventional television sync signals.
An assembly 2 of artificial persistence or afterglow circuits, whose function is to produce for the digital information stored in memory 4 for which there is no modification due to aging, a persistence effect comparable to that produced on a tube where the brightness of a point starts to decrease as soon as it is written.
An assembly 8 of control circuits, e.g. realized by means of a microprocessor, receives both the radar video signals and the rotation signals and ensures the control and synchronization of all the aforementioned assemblies, apart from the monitor 7.
Thus, after converting the scanning standard (polar into Cartesian), the image memory 4 contains the image as it will be displayed on the television monitor 7, this image then being called the basic image.
The problem which is often encountered by the operator is that of wishing to magnify one or more areas of said basic image, where an important detail requires examination. These magnified areas are called "insets".
FIGS. 2a and 2b show an example of the display of an area monitored by a radar. In FIG. 2a, a circle indicates the range P of a radar located at a point C.sub.o, forming the center of the circle P. Of the area P covered by the radar, only a part is displayed in the form of an image I as shown in FIG. 2b. For example, the displayed part in FIG. 2b shows an airport having runways and taxiways along which are travelling two aircraft A.sub.1 and A.sub.2.
In exemplified manner, it is possible to magnify those areas of the basic image I surrounding aircraft A.sub.1 and A.sub.2 to respectively form insets M.sub.2 and M.sub.3 which are preferably inserted in those parts of the basic image I not useful to the operator. It should be noted that the surface of the magnified parts and their magnification are not necessarily the same for each inset. Moreover, still in exemplified manner, an inset M.sub.1 is inserted in image I from part of the area P covered by the radar not included in the displayed portion I. The surface and magnification coefficient of said inset M.sub.1 are also of a random nature compared with those of other insets.
It is known to obtain such a partial magnification by expansion, during the reading of the image memory, of an area of the image supplied by the memory, each of the points intended to be part of the inset then being repeated identically thereto k times, if k is the sought magnification coefficient. This method is known as "zoom". It has the drawback of not improving the definition of the observed magnified area and consequently the overall definition of the inset is very inferior to that of the remainder of the basic image.